Government doesn't need oversupply.

It is just historically rural regions in the US farmed corn, and it was enough for the US and little bit of export.

But over time agriculture effectiveness has significantly improved due to: fertilizer technology (CF Industries), agricultural equipment (John Deere), seeds technology (Monsanto Bayer Dow Chemical), food processing.

Thanks to that, the yield has significantly improved and amount of corn produced has increased dramatically, even without increasing land

Think of Moore's Law, but applied to food instead of microtransistors